It is well known in the art that adding branching agents to polyesters will increase their melt strength. However, to date, there are very few successful commercial branched polyesters. One drawback of branched polyesters is the tendency to form gels in the application of extrusion into films. These gels are localized, higher molecular weight areas giving rise to an uneven, poor clarity appearance to the film. One common solution is to equip extruders with screws having high shear or high mixing capabilities. This forces the users to install new screws for their existing machines and thus incur extra expense to the end users. The problem with gels can be alleviated by introducing a controlled amount of moisture (as vapors of water, alcohols or glycols and in general, any organic compound with one or more hydroxyl groups) into the effluent gas (e.g. nitrogen, helium, argon and the like or mixtures there of including air) during solid-state polymerization of the polyesters, and films with high clarity can be obtained with regular extrusion screws.
High molecular weight polyesters are generally prepared by a melt-phase polycondensation process followed by a solid-state polymerization process. When the polyester is branched, the chain branching often results in an increase in the tendency for chain entanglement. Areas of high entanglement may be viewed as localized networks which are the centers of gels observed on a macro scale upon molding into articles.
During melt phase polymerization, polymer chains have enough mobility so that localized networks due to entanglement are reduced. On the contrary, during solid-state polymerization, parts of the polymer chains are already engaged in crystalline regions. Therefore, the chains have limited mobility. As the polymerization and cross linking progresses, `knots` and `kinks` are formed. The degree of entanglement is therefore increased leading to the formation of localized polymer networks. These entanglements manifest themselves as gels which are often observed upon molding into articles. The greater the difference between solid-state polymerized and melt-phase polymerized molecular weight, the greater the tendency to form gels.
Parts of the polymer chains in the localized polymer networks formed during solid-state polymerization are under excessive strain because of the lack of mobility of the chains. The points along the polymer chain with the highest strains are most vulnerable to chemical attack. If certain molecules are present in the effluent gas during solid-state polymerization, the bonds under the greatest strain will be broken the fastest, thus producing a `relaxing` effect for the networks and entanglement is reduced. As a result, the number of gels is reduced.
Applicants are not aware of any prior disclosures of adding vapor of compounds having reactive OH groups to the effluent gas to produce branched polyesters of high molecular weights that will reduce gels and result in high clarity articles. In fact, most of the literature advises that moisture should be kept to a minimum to prevent loss of solid-state polymerization rate. There are a few patents that mention use of water/alcohols with solid-state polymerization, but they are irrelevant to the present invention, as explained below. Treatment of poly(ethylene terephthalate) polymers with supercritical carbon dioxide and water under high pressure to reduce acetaldehyde is known. However, both carbon dioxide and high pressure are required and it was not under solid-state polymerization conditions. Swiss Patent Application No. 655,938 discloses a process of two stages: (1) treating the polyester with alcohol or alcohol/water until the aldehyde content is below 35 ppm, then (2) postcondensing at 200.degree.-245.degree. C. It requires the measurement for acetaldehyde at the first stage and is therefore irrelevant to our present invention which applies to polymers without the need of intermediate measurement of acetaldehyde. Japanese Patent No. 59219328 discloses a process to perform moisture conditioning with a moisture content of at least 0.2 wt % to reduce acetaldehyde. The level of water disclosed is much higher than that which our present invention requires and is therefore irrelevant. Japanese Patent No. 55013715 discloses extraction of polyesters before or after solid-state polymerization by dipping the polyesters in solvents. European Patent Application No. 389,948 discloses bringing PET having an intrinsic viscosity of at least 0.50 dl/g and a density of 1.38 or more into contact with water to reduce the amounts of oligomers and acetaldehyde formed at the time of molding.
The present invention involves contacting the precursor polyester particles with the vapor of water or an organic compound having one or more hydroxyl groups, preferably into the effluent gas during or after solid-state polymerization of polyesters including during crystallization.